West Nile Virus and Other Arboviral Diseases — United States, 2012

Arthropod-borne viruses (arboviruses) are transmitted to humans primarily through the bites of infected mosquitoes and ticks. West Nile virus (WNV) is the leading cause of domestically acquired arboviral disease in the United States. However, several other arboviruses also cause sporadic cases and seasonal outbreaks of neuroinvasive disease (e.g., meningitis, encephalitis, and acute flaccid paralysis). In 2012, CDC received reports of 5,780 nationally notifiable arboviral disease cases (excluding dengue). A large multistate outbreak of WNV disease accounted for 5,674 (98%) of reported cases, the highest number reported since 2003. Other reported etiologies included Eastern equine encephalitis virus (EEEV), Powassan virus (POWV), St. Louis encephalitis virus (SLEV), and California serogroup viruses such as La Crosse virus (LACV) and Jamestown Canyon virus (JCV). Arboviruses continue to cause serious illness in substantial numbers of persons in the United States. Maintaining surveillance remains important to identify outbreaks and guide prevention efforts.


West Nile Virus and Other Arboviral Diseases -United States, 2012
Arthropod-borne viruses (arboviruses) are transmitted to humans primarily through the bites of infected mosquitoes and ticks. West Nile virus (WNV) is the leading cause of domestically acquired arboviral disease in the United States (1). However, several other arboviruses also cause sporadic cases and seasonal outbreaks of neuroinvasive disease (e.g., meningitis, encephalitis, and acute flaccid paralysis) (1). In 2012, CDC received reports of 5,780 nationally notifiable arboviral disease cases (excluding dengue). A large multistate outbreak of WNV disease accounted for 5,674 (98%) of reported cases, the highest number reported since 2003. Other reported etiologies included Eastern equine encephalitis virus (EEEV), Powassan virus (POWV), St. Louis encephalitis virus (SLEV), and California serogroup viruses such as La Crosse virus (LACV) and Jamestown Canyon virus (JCV). Arboviruses continue to cause serious illness in substantial numbers of persons in the United States. Maintaining surveillance remains important to identify outbreaks and guide prevention efforts.
In the United States, most arboviruses are maintained in transmission cycles between arthropods and vertebrate hosts (typically birds or small mammals). Humans usually become infected when bitten by infected mosquitoes or ticks. Person-to-person transmission occurs rarely through blood transfusion and organ transplantation. The majority of human arboviral infections are asymptomatic. Symptomatic infections most often manifest as a systemic febrile illness and, less commonly, as neuroinvasive disease. Most endemic arboviral diseases are nationally notifiable and are reported to CDC through ArboNET (2,3). In addition to collecting data on human disease cases, ArboNET collects data on viremic blood donors, veterinary disease cases, and infections in mosquitoes, dead birds, and sentinel chickens. Using standard definitions, human cases with laboratory evidence of recent arboviral infection are classified as neuroinvasive disease or nonneuroinvasive disease (2). Because of the substantial associated morbidity, detection and reporting of neuroinvasive disease cases is assumed to be more consistent and complete than for nonneuroinvasive disease cases. Therefore, incidence rate calculations were limited to neuroinvasive disease cases.
Seven POWV neuroinvasive disease cases were reported from three states: Minnesota (four cases), Wisconsin (two), and New York (one) ( Table 1). Dates of illness onset for all cases were in May or June. All cases occurred in adult patients (median age: 58 years [IQR: 36-73 years]); four were male. Six (86%) patients were hospitalized; none died.
Three SLEV disease cases were reported from Texas; only one was neuroinvasive. Dates of illness onset were in July and August. All cases occurred in adults aged 40-60 years. One of the three SLEV patients was hospitalized; none died.

Editorial Note
A large multistate outbreak of WNV disease occurred in 2012, with more cases reported nationally than in any year since 2003, including the first reported human case from Maine. The 15 EEEV disease cases reported in 2012 were the most reported since 2005, and included the first cases ever reported from Vermont. EEEV disease remained the most severe domestic arboviral disease, with a 33% case-fatality rate. Over 90% of arboviral disease cases occurred during July-September, and most of the remainder occurred during April-June, emphasizing the importance of focusing public health interventions on these periods.
Puerto Please note: Errata have been published for this issue. To view the errata, please click here and here.
The national incidence of WNV neuroinvasive disease peaked in 2002 (1.02 per 100,000) and 2003 (0.98) (3). During 2004-2011, annual incidence was relatively low (median: 0.31; range: 0.13-0.50) (3)(4)(5)(6). In 2012, the national incidence of WNV neuroinvasive disease increased to 0.92 per 100,000. The increase in disease was widespread, with 43 states reporting a higher incidence in 2012 compared with the median for 2004-2011; however, more than half of the neuroinvasive disease cases in 2012 were reported from just four states, and 29% were reported from Texas alone. Of the five states with a lower incidence in 2012 compared with the previous 8 years, four were in the Mountain Region (Montana, Nevada, Utah, and Wyoming). Oregon also reported lower rates compared with recent years; Alaska and Hawaii have never reported a case of WNV disease.
Reported numbers of arboviral disease cases vary from year to year. It is not clear why more WNV activity occurred this year than in recent years. The weather, numbers of birds that maintain the virus, numbers of mosquitoes that spread the virus, and human behavior are all factors that can influence when and where outbreaks occur. Because of this complex ecology, it is difficult to predict how many cases of disease might occur in the future and in what areas.
The findings in this report are subject to at least two limitations. First, ArboNET is a passive surveillance system that relies on clinicians to consider the diagnosis of an arboviral disease and obtain appropriate diagnostic test results and on healthcare providers and laboratories to report laboratory-confirmed cases to public health authorities. Second, testing and reporting are incomplete, leading to a substantial underestimate of the actual number of cases (7). Based on previous studies, for every reported case of WNV neuroinvasive disease, there are an estimated 30-70 nonneuroinvasive disease cases. Extrapolating from the 2,873 WNV neuroinvasive disease cases reported, an estimated 86,000-200,000 nonneuroinvasive disease cases might have occurred in 2012. However, only 2,801 (1%-3%) were diagnosed and reported.
Arboviruses continue to cause severe illness in substantial numbers of persons in the United States. However, cases occur sporadically, and the epidemiology varies by virus and geographic area. Surveillance is essential to identify outbreaks and guide prevention efforts aimed at reducing the incidence of these diseases. Health-care providers should consider arboviral infections in the differential diagnosis of cases of aseptic meningitis and encephalitis, obtain appropriate specimens for laboratory testing, and promptly report cases to public health authorities (2). Because human vaccines against domestic arboviruses are not available, prevention of arboviral disease depends on community and household efforts to reduce vector populations (e.g., applying insecticides and reducing mosquito breeding sites), personal protective measures to decrease exposure to mosquitoes and ticks (e.g., use of repellents and wearing protective clothing), and screening blood donors. Updated guidelines for WNV surveillance, prevention, and control are available online from CDC at http://www.cdc.gov/westnile/ resources/pdfs/wnvguidelines.pdf.
What is already known on this topic? West Nile virus (WNV) is the leading cause of domestically acquired arboviral disease in the United States. However, several other arboviruses can cause sporadic cases and outbreaks of neuroinvasive disease, mainly in the summer.
What is added by this report?
A large multistate outbreak of WNV disease occurred in 2012. The 5,674 cases reported nationally were the highest number of cases reported since 2003. Eastern equine encephalitis, although rare, remained the most severe arboviral disease, with a 33% case-fatality rate.
What are the implications for public health practice?
WNV and other arboviruses continue to be a source of severe illness each year for substantial numbers of persons in the United States. Maintaining surveillance remains important to identify outbreaks and guide prevention efforts.